A. Nanoparticles
There has been substantial interest in the preparation and characterization of compound semiconductors consisting of particles with dimensions in the order of 2-100 nm, often referred to as quantum dots and/or nanoparticles. These studies have focused mainly on the size-tunable electronic, optical and chemical properties of nanoparticles. Semiconductor nanoparticles are gaining substantial interest due to their applicability for commercial applications as diverse as biological labeling, solar cells, catalysis, biological imaging, and light-emitting diodes.
Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are primarily responsible for their unique properties. The first is the large surface-to-volume ratio: as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor is that, with many materials including semiconductor nanoparticles, the electronic properties of the material change with size. Moreover, because of quantum confinement effects, the band gap typically gradually becomes larger as the size of the nanoparticle decreases. This effect is a consequence of the confinement of an ‘electron in a box,’ giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material. Semiconductor nanoparticles tend to exhibit a narrow bandwidth emission that is dependent upon the particle size and composition of the nanoparticle material. The first excitonic transition (band gap) increases in energy with decreasing particle diameter.
Semiconductor nanoparticles of a single semiconductor material, referred to herein as “core nanoparticles,” along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface that can lead to non-radiative electron-hole recombinations.
One method to eliminate defects and dangling bonds on the inorganic surface of the quantum dot is to grow a second inorganic material, typically having a wider band-gap and small lattice mismatch to that of the core material, on the surface of the core particle, to produce a “core-shell” particle. Core-shell particles separate carriers confined in the core from surface states that would otherwise act as non-radiative recombination centers. One example is ZnS grown on the surface of CdSe cores. Another approach is to prepare a core-multi shell structure where the “electron-hole” pair is completely confined to a single shell layer consisting of a few monolayers of a specific material such as a quantum dot-quantum well structure. Here, the core is of a wide bandgap material, followed by a thin shell of narrower bandgap material, and capped with a further wide bandgap layer. An example is CdS/HgS/CdS grown using substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayers of HgS that is then over grown by monolayers of CdS. The resulting structures exhibit clear confinement of photo-excited carriers in the HgS layer.
The most studied and prepared semiconductor nanoparticles have been II-VI materials, for example, ZnS, ZnSe, CdS, CdSe, and CdTe, as well as core-shell and core-multi shell structures incorporating these materials. Other semiconductor nanoparticles that have generated considerable interest include nanoparticles incorporating III-V and IV-VI materials, such as GaN, GaP, GaAs, InP, and InAs. Methods of synthesizing core and core-shell nanoparticles are disclosed, for example, in co-owned U.S. Pat. Nos. 6,379,635, 7,803,423, 7,588,828, 7,867,556, and 7,867,557. The contents of each of the forgoing patents are hereby incorporated by reference, in their entirety.
B. Surface Modification
Many applications of nanoparticles require that the semiconductor nanoparticle be compatible with a particular medium. For example, some biological applications such as fluorescence labeling, in vivo imaging and therapeutics require that the nanoparticles be compatible with an aqueous environment. For other applications, it is desirable that the nanoparticles be dispersible in an organic medium such as aromatic compounds, alcohols, esters, or ketones. For example, ink formulations containing semiconductor nanoparticles dispersed in an organic dispersant are of interest for fabricating thin films of semiconductor materials for photovoltaic (PV) devices.
A particularly attractive potential field of application for semiconductor nanoparticle is in the development of next generation light-emitting diodes (LEDs). LEDs are becoming increasingly important, in for example, automobile lighting, traffic signals, general lighting, and liquid crystal display (LCD) backlighting and display screens. Nanoparticle-based light-emitting devices have been made by embedding semiconductor nanoparticles in an optically clear (or sufficiently transparent) LED encapsulation medium, typically a silicone or an acrylate, which is then placed on top of a solid-state LED. The use of semiconductor nanoparticles potentially has significant advantages over the use of the more conventional phosphors. For example, semiconductor nanoparticles provide the ability to tune the emission wavelength of a LED. Semiconductor nanoparticles also have strong absorption properties and low scattering when the nanoparticles are well dispersed in a medium. The nanoparticles may be incorporated into an LED encapsulating material. It is important that the nanoparticles be well dispersed in the encapsulating material to prevent loss of quantum efficiency. Methods developed to date are problematic because the nanoparticles tend to agglomerate when formulated into LED encapsulants, thereby reducing the optical performance of the nanoparticles. Moreover, even after the nanoparticles have been incorporated into the LED encapsulant, oxygen can still migrate through the encapsulant to the surfaces of the nanoparticles, which can lead to photo-oxidation and, as a result, a drop in quantum yield (QY).
A nanoparticle's compatibility with a medium as well as the nanoparticle's susceptibility to agglomeration, photo-oxidation and/or quenching, is mediated largely by the surface composition of the nanoparticle. The coordination about the final inorganic surface atoms in any core, core-shell or core-multi shell nanoparticle is incomplete, with highly reactive “dangling bonds” on the surface, which can lead to particle agglomeration. This problem is overcome by passivating (capping) the “bare” surface atoms with protecting organic groups, referred to herein as capping ligands or a capping agent. The capping or passivating of particles not only prevents particle agglomeration from occurring. The capping ligand also protects the particle from its surrounding chemical environment and provides electronic stabilization (passivation) to the particles, in the case of core material. The capping ligand is usually a Lewis base bound to surface metal atoms of the outer most inorganic layer of the particle. The nature of the capping ligand largely determines the compatibility of the nanoparticle with a particular medium. These capping ligands are usually hydrophobic (for example, alkyl thiols, fatty acids, alkyl phosphines, alkyl phosphine oxides, and the like). Thus, the nanoparticles are typically dispersed in hydrophobic solvents, such as toluene, following synthesis and isolation of the nanoparticles. Such capped nanoparticles are typically not dispersible in more polar media.
The most widely used procedure to modify the surface of nanoparticles is known as ligand exchange. Lipophilic ligand molecules that coordinate to the surface of the nanoparticle during core synthesis and/or shelling procedures may subsequently be exchanged with a polar/charged ligand compound. An alternative surface modification strategy intercalates polar/charged molecules or polymer molecules with the ligand molecules that are already coordinated to the surface of the nanoparticle. Current ligand exchange and intercalation procedures may render the nanoparticle more compatible with aqueous media but usually result in materials of lower quantum yield (QY) and/or substantially larger size than the corresponding unmodified nanoparticle.
Thus, there is a need in the art for nanoparticles that are compatible with a variety of media and for techniques for modifying the surface of nanoparticles to render desired compatibility while maintaining the integrity and photophysical properties of the nanoparticle.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.